1. Field of the Invention
The present invention relates to a brushless motor driving device, a brushless motor starting method, and a method of detecting the rotor stop position of a brushless motor.
Priority is claimed on Japanese Patent Application No. 2006-206390, filed Jul. 28, 2006, Japanese Patent Application No. 2006-242647, filed Sep. 7, 2006, and Japanese Patent Application No. 2007-000469, filed Jan. 5, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
In a brushless motor of the type in which the rotor has a permanent magnet, position sensorless drive control may be performed in which position sensors that detect the rotational position of the rotor are not provided. In this case, the rotational position of the rotor is detected from the edge gap of a pulse signal obtained by inputting an induced voltage that appears at the motor terminals of an open section (non-energized phase) and an equivalent neutral-point potential to a comparator. However, when the rotational frequency is zero or the rotational frequency is extremely low, such as during startup of a brushless motor, since the induced voltage is not generated or is extremely small, a signal sufficient for detection of the rotational position is not acquired.
A conventional method for detecting the stop position of a rotor includes detecting the voltage impressed on coils of three phases and detecting the inductances of the coils from the differences in voltage rise times to determine the coils facing magnetic poles of permanent magnets (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2004-40943). In the event of the flux direction of the coil and the flux direction of the magnet core not coinciding, when a current is flowed, the impedance changes before and after the flow of the current due to residual magnetization of the magnet core. Therefore, in the driving device that is disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-40943, a current is flowed in succession two times or more in the same phase and the coil with the smallest voltage rise time in the second time onward is detected.
As another method of detecting the rotor stop position, three-phase conduction is performed by simultaneously flowing short pulse currents, a degree of which does not rotate the rotor, from one coil to the two other coils. By measuring the pulse width of square-wave pulse voltages that are simultaneously generated in the two coils when the current is turned OFF, the rotor stop position is determined (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2002-335691). Since the pulse widths of the two square-wave pulse voltages that are simultaneously generated in the two coils subtly change in accordance with differences in the rotor stop position, the rotor stop position is specified by comparing both.
Also, as a conventional method of starting a brushless motor, open loop forced energization is performed that forcibly switches the energization without detecting the rotor position, and when the rotor position can be detected, the energization switching is controlled based on the aforementioned pulse signal (for example, refer to Japanese Unexamined Utility Model Application, First Publication No. H06-25224).
In addition, there is also a method of starting a brushless motor which includes putting the rotor position in a locked state by attracting the rotor position to a specific position by applying a first energizing pattern that positions the rotor at a specific position, applying a second energizing pattern that leads the first energizing pattern by 60° for a minimal time, and then applying a third energizing pattern that leads the second energizing pattern by a further 60° (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2001-211684).
Here, as a starting method in the case of the brushless motor rotating in reverse by an external load, there is the method disclosed in Japanese Unexamined Patent Application, First Publication No. 2001-128485. First, after reducing the rotational frequency of the rotor by three-phase energization, the energization during the three-phase energization is repeatedly turned ON/OFF, and the phase difference and the position at which the phase current becomes zero are investigated. The rotational direction is determined from the phase difference of the currents, and in the case of being determined to be in a reverse rotational state, the reverse rotational frequency is found from a generation cycle of zero point at each phase. After an AC current with a reverse rotational frequency is supplied and brought in, the frequency is slowly changed from a reverse direction to a frequency in a forward direction, whereby the motor can be started by raising the rotational frequency of the rotor to a desired rotational frequency.
However, when detecting the stop position of a rotor, in the method as disclosed in Japanese Unexamined Patent Application, First Publication No. 2004-40943, it is necessary to add transistors, resistors, and comparators and the like that constitute a stop position detecting circuit that detects the rotor stop position, thus becoming a factor in complicating the device constitution. Also, as the difference between voltage rise times is small, the inductance difference becomes minute, thereby hindering an accurate determination.
The method disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-335691 has the advantage of eliminating special-purpose circuits, but since the difference between inductances due to the impressed voltage in the three-phase energization is small, the difference between pulse widths of square-wave pulse voltages becomes small, thereby hindering an accurate detection.
Also, among conventional methods of starting a brushless motor, in the case of open loop forced energization being performed, its susceptibility to disturbances causes a high parameter dependency of the brushless motor, which makes the setting of the parameters difficult. Also, continuing forced energization slowly causes the rotor to start to rotate. As a result, the time required for starting the motor becomes longer, and the torque during starting diminishes. Performing forced energization switching many times further increases the susceptibility to disturbances.
In the case of the rotor being put in a locked state by a predetermined energization pattern, and also in the case of measuring inductance, since it is necessary to perform open loop forced energization during the period the brushless motor starts to rotate until the rotor position can be detected, the same issues are involved. Moreover, in the case of putting the rotor in a locked state, in a motor with large inertia, a long time is required for positioning the rotor.
In the case of starting from a reverse rotational state, since the rotational direction and the frequency of rotation are calculated after finding the position at which the phase current becomes zero, complicated processing is required. Also, by slowly changing the frequency from a reverse rotational direction to a frequency in a forward direction, a long time is required until starting.
The present invention was achieved in view of the above circumstances, and has as its main object to start a motor in a short time by a simple method and also to enable high torque to be obtained during starting.